=Arc of recess.= That part in the revolution of a pair of gear wheels in which the teeth in contact recede from the line of centres of the two wheels.

=Arc-pitch.= The pitch of gear wheel teeth when measured around the pitch circle.

=Attachment.= A work-holding device that may be attached to a machine.

=Auger.= A wood-boring tool having two spiral plates and a pointed screw to feed it, the cutting edge being at the end of the tool.

=Axle-box.= The bearing in which an axle revolves.

B

=Back-gear.= The toothed wheels on the spindle of a lathe and at the back of the lathe-head, by means of which the speed of the lathe is reduced.

=Back-geared lathe.= A lathe having a back gear to reduce its motion.

=Back-knife gauge-lathe.= A lathe in which the work is finished and cut to size and shape by a knife at the back of the lathe.

=Balanced pulley.= A pulley whose weight is so equally distributed that it will run steadily and smoothly at the speed for which it is balanced.

=Balanced valve.= A valve so constructed as to move with equal force in either direction.

=Ball and socket joint.= A universal joint consisting of a ball on the end of a shaft and in a casing that envelops it and yet permits it to be moved in its casing.

=Ball-cutter.= A tool for finishing metal b.a.l.l.s.

=Ball-pene.= A spherical pene of a hammer.

=Band-saw.= A continuous ribbon of steel having saw teeth on one of its edges.

=Band-saw machine.= A machine for operating a band-saw.

=b.a.s.t.a.r.d file.= A file whose teeth are one degree or grade coa.r.s.er than a _second_ cut file and one degree finer than a _coa.r.s.e_ cut file.

=Belt.= A leather band employed to drive pulleys, for transmitting motion.

=Belt-clamp.= A clamp for pulling the ends of a belt together, to lace it, while the belt is upon the pulleys.

=Belt-hook.= A hook employed to fasten the ends of belts together.

=Belt-pulley.= A wheel that drives or is driven by a belt.

=Belt-shipper.= A device for moving a belt from one pulley to another.

=Belt-tightener.= A pulley employed to cause a belt to tighten upon another pulley to enable it to transmit motion periodically instead of continuously.

=Bevel-sawing machine.= A wood-working machine in which the saw or the work table may be set to cut a surface at other than a right angle to the face of the work that rests against the work table or the fence as the case may be.

=Bevel-square.= A square whose blade may be set to any required angle to the stock that holds it.

=Bevel-wheel=, _or_ =bevel-gear.= A gear wheel with its teeth at an angle to its shaft.

=Bit.= 1. A boring tool. 2. A tool that is carried in a holder.

=Blank.= A piece of material roughly formed and ready to be formed into some definite shape.

=Blast-pipe.= 1. The pipe conveying the blast or air to a fire furnace or cupola. 2. A small pipe through which steam escapes up a locomotive chimney to increase the draught of the fires.

=Blob.= An extremely loose place in a plate or saw blade.

=Block-plane.= A short plane.

=Boiler-sh.e.l.l.= The outer casing of a steam-boiler.

=Bolt.= 1. A holding device having a head at one end and at the other a threaded stem to receive a nut. 2. A short piece of a round log.

=Bolt-cutter.= A machine for cutting screw threads upon bolts or similar work.

=Boring-bar.= A bar that carries boring tools.

=Boring-machine.= A machine for boring holes in metal or wood.

=Boring-mill.= A form of lathe used mainly for boring.

=Boring-tool.= A tool for cutting out and enlarging a bore or hole.

=Boss.= An enveloping piece on an axle or shaft and having upon it an arm, arms, or spokes.

=Bottoming-tap.= A tap having a full thread up to its very end so that it will cut a full thread to the bottom of a hole.

=Box-chuck.= A rectangular two-jawed chuck used by bra.s.s finishers.

=Box-tool.= A tool used in screw machines and turret heads, and which guides the work while it is being operated upon. A box tool in many cases carries more than one cutting tool.

=Box-wrench.= A wrench which fits over the head of the bolt and pa.s.ses endways upon it.

=Brace.= 1. A rod, bar, or beam that braces or supports. 2. A device for revolving cutting tools.

=Bracket.= A projecting frame that is bolted to its supporting pieces or frame.

=Brad-awl.= An awl for piercing small holes in wood and having a wedge-shaped end.

=Branch-pipe.= A pipe leading out of another.

=Bra.s.s-and-bra.s.s.= A term used to denote that the two bra.s.ses or boxes of a bearing are locked together by the key, cap, or set-screw.

=Bra.s.ses.= Pieces fitted into a frame and intended to afford a bearing for a journal.

=Break-lathe= _or_ =gap-lathe.= A lathe having a break or gap in the bed and beneath the face plate to let chucked work of large diameter pa.s.s.